The problems associated with the lubrication of gears such as utilized in automotive transmissions and axles are well known to those skilled in the art. In the lubrication of automatic transmissions, proper fluid viscosity at both low and high temperatures is essential to successful operation. Good low temperature fluidity eases cold weather starting and insures that the hydraulic control system will properly "shift gears". High viscosity at elevated temperatures insures pumpability and the satisfactory operation of converters, valves, clutches, gears and bearings. These conflicting fluidity requirements require a product that exhibits the following characteristics:
(a) high temperature viscosity retention,
(b) low temperature fluidity,
(c) shear stability, and
(d) high temperature stability.
In order to prepare lubricants having these characteristics, it has become common practice to add a variety of chemicals to the lubricating oil. For example, in order to meet the viscosity requirements, compositions have been added to the oils which are characterized by relatively small change in their viscosity with changing temperature. In general, lubricants containing such compositions have the desirable properties of functioning immediately, even though cold, upon being put into service, and to continue to function satisfactorily as they become heated during operation. Commonly used gear oil viscosity improvers include polymethacrylates and polyolefins.
In addition to viscosity improvers, lubricating compositions useful as gear lubricants generally will contain pour point depressants, extreme pressure agents, oxidation inhibitors, corrosion inhibitors, foam inhibitors, and friction modifiers.
Lubricating compositions have been suggested containing various nitrogen-containing and phosphorus-containing compositions to impart desirable properties to lubricating compositions. For example, U.S. Pat. No. 3,513,093 describes lubricant compositions containing substituted polyamines which comprise the reaction product of an alkylene amine with a substantially hydrocarbon-substituted succinic acid and at least about 0.001 mole of a phosphorus acid-producing compound selected from the group consisting of phosphoric acids, phosphorous acids, phosphonyl acids, phosphinyl acids, and the esters, the halides and the anhydrides thereof. The substituted polyamines are useful as anti-wear agents, anti-rust agents, detergents, etc. U.S. Pat. No. 4,338,205 describes a lubricating oil with improved diesel dispersancy. The lubricating oils contain an acid-treated, oil-soluble alkenyl succinimide or a borated alkenyl succinimide which has been treated at an elevated temperature with an oil-soluble strong acid such as an alkyl sulfonic acid, or a phosphoric acid. The oil-soluble organic acids are generally classified as those acids containing a hydrogen-phosphorus moiety which has a pK of -10 to about +5.0.
More recently, new demands are being placed on lubricants to be used in gear applications. Increases in commercial vehicle power and loading require the lubricant to be able to withstand severe thermal stressing while protecting the equipment being lubricated. Thus, the high temperature stability (e.g., above about 160.degree. C.) of lubricants designed for gear applications is a significant consideration.